The present invention relates to an optical fibre hydrophone. The invention relates to the field of acoustic detection in a marine medium and more particularly to a monomodal optical fibre hydrophone operating by the elastooptical effect, using interaction effects between the sound waves to be detected and a monomodal optical fibre on which said sound wave acts.
The invention relates to a hydrophone more particularly having a monomodal optical fibre which is immersed in the water, where the sound wave is propagating. The propagation of this sound wave produces pressure variations in the propagation medium and these lead to geometrical and optical parameter variations in the fibre due to the elastooptical effect. An optical wave propagating in the fibre undergoes phase variations, which can be detected by interferometry using a second monomodal optical fibre forming the reference arm. The optical connections between the optical fibre immersed in the water and which constitutes the measuring arm and the reference optical fibre are established in an optical structure forming an interferometer designed according to the Michelson interferometer principle. The Michelson interferometer generally comprises a monochromatic radiation source, an optical beam splitter such as a semitransparent plate which supplies two measuring arms terminated by mirrors and a radiation detector for collecting in superimposed manner via the optical splitter, the radiation which has performed an outward and return path along the two measuring arms. Such a device makes it possible to measure a large number of physical quantities able to affect the propagation of optical radiation along the measuring arms. Certain of these physical quantities lead to reciprocal effects producing the same transmission delay, no matter what the propagation direction of the optical radiation in each of the measuring arms. Other physical quantities cause non-reciprocal effects, which influence the transmission delay differently as a function of the propagation direction of the optical radiation. The two non-reciprocal effects which are conventionally considered are the Faraday effect and the relativistic inertial effect. The Faraday effect occurs when the measuring arm has a material medium in which a magnetic field produces a preferred electron spin orientation. The relativistic inertial effect used in a ring-type interferometer is called the Sagnac effect and the interferometer is then called a gyrometer.
The reciprocal effects are not linked with the destruction of the symmetry of the space or a material medium. They are observed when the measuring arms are the source of optical, thermal or mechanical stresses.
In addition, optical reflection systems based on the use of photorefractive media are known, which make it possible to reflect an incident wave front in the form of a conjugate wave front. An ordinary mirror reflects the light if it came from a virtual object not coinciding with the object illuminating said mirror. Conversely, a photorefractive medium can reflect a wave front having a conjugate space, which returns to the object a radiation isomorphic of that coming therefrom. In the presence of reciprocal effects, this interactive reflection ensures the insensitivity to such effects, provided that they have not varied during the outward and return path of the radiation and the photorefractive medium has been able to adapt to the variations of these effects. However, this aptitude to cancel out the reciprocal effects is not prejudicial to the interferometric measurement of the non-reciprocal effects.
U.S. Pat. No. 4,320,475 describes a first prior art device comprising a monomodal optical fibre hydrophone operating by the elastooptical effect, which has a monomodal laser source coupled to an integrated optical circuit board in which are realized means for splitting the radiation from the source towards two first integrated optical guides and means for recombining the radiation guided by two second integrated optical guides, a first monomodal fibre forming a measuring arm which is to be immersed in the interaction medium exposed to the sound wave to be detected, and a second monomodal optical fibre forming a reference arm, said two fibres having their ends respectively rigidly coupled to one of the first and one of the second optical guides the hydrophone also has an electrically controlled phase modulator acting on one of the integrated optical guides in order to introduce a phase displacement between the guided waves, means for detecting the radiation from the recombination means and a device for processing the detected signals for supplying a control signal to the phase modulator, such that the phase displacement between the two arms an independent of the sound wave to be detected is maintained close to a maximum sensitivity point. The processing device also supplies a measuring signal which is characteristic of the sound waves.
Moreover, European patent application No. 0 079 268 describes a Michelson interferometer in which the mirrors are replaced by a conjugate mirror. This interferometer is sensitive to non-reciprocal effects. This mirror is said to be "conjugate", in fact an incident divergent wave in the medium is reflected in the form of a convergent wave conjugate of the incident wave. This photorefractive mirror Michelson interferometer comprises a monochromatic radiation source, an optical beam splitting means supplying two fractions of said radiation to two measuring arms terminating by reflecting means and a photodetector, the latter being arranged so as to collect in superimposed manner said two radiation fractions having performed an outward and return travel in the two arms.
The two radiation fractions interfere within this photorefractive medium, a reflector then making it possible to reflect under normal incidence towards said medium the first radiation fraction which is passed through it. However, this second prior art device using the conjugate wave front of the first incident wave fraction reaching the photorefractive medium requires that the reflected wave is the conjugate wave of the incident wave and consequently that the medium does not deform the wave front of the incident wave.
Compared with the first prior art device, the hydrophone according to the invention constitutes a high-pass frequency filter and makes it possible to eliminate the consequences of low frequency interference. Moreover, it permits a self-alignment of the conjugate beam, which is recoupled in the fibre without requiring any alignment. It also permits a remote supply in the interaction medium, which makes it undetectable by conventional detection means, such as sonar.